Devices for creating patterns of various type on paper supports, or the like, for many purposes are known. A common case are devices for creating perforation lines that serves as tear-off lines dividing a printed form starting from adjacent forms, in a continuous printing process or operation, or as tear-off lines of a same document, in order to separate document portions that have different destinations.
The perforations are made mechanically, for example by an apparatus as described in ITPI2008A000075. However, this technique has some drawbacks. In particular, tear lines with a limited number of shapes can be created. Furthermore, cut or perforation blades must be sharpened often.
Methods are also known for creating pattern on paper material or the like by projecting a laser beam on it. By these methods, even complicated engravings can be created such as drawings or lacework, but the precision is acceptable only if the support is still or substantially still while it is engraved. In the case of targets like isolated three-dimensional bodies, or fabric or sheets of planar material, the target must be arranged in a working area, and then a laser engraving head is operated, like a movable head, for instance a two or more axes robot head, or a fixed head, which can comprise galvanometer-based lenses and mirrors.
In the case of laser machines for engraving on a web of paper or plastics, the web must be stopped when perforation lines or patterns are engraved, and must be started again once the engraving has been performed.
It is also possible to carry out laser engravings on a slow-moving leather or fabric web. In this case, amounts of material are removed by directing laser pulses along a line transversal to the web, with a slight inclination, in order to take into account the web conveying speed, even if this is very slow.
With reference to laser engraving systems that are configured for orienting a laser beam by means of galvanometers that actuate oscillating mirrors, an example is disclosed in WO9847035. These systems can be used for creating any engravings, in particular they are well-suited for engraving inscriptions and numbers.
These systems are suitable for engraving complicated pattern on a still support at a high speed. However, due to the speed at which the galvanometer-operated mirrors and lenses are moved, it is particularly difficult to accurately reproduce the intersection of graphic elements that meet forming sharp angles. For instance, this is the case of most types of the commonly used fonts. Due to the high speed of the support, beveled or irregular vertices, i.e. connections between the graphic elements, are formed instead of the sharp angles. This may lead to unacceptable distortions that affect an optical reading of the engraved patters, in particular alphanumeric symbols and other graphic symbols.
Another problem of the known art for creating patterns on supports or webs of paper, as well as of other materials, the choice of the power that the laser system supplies to the support is critical. In particular, the use of the laser for marking documents involves a risk of burning the support. For this reason, an accurate control of the power of the laser engraving beam is required. Since the laser emitter is excited, the power has a transient behavior, with the consequence that the shorter the laser emission of a single engraving event, the stronger the effect of the transient behavior that negatively affects the engraving quality. More in detail, the inner working temperature of the emitter is reached by the laser in a time that depends upon the power that the emitter is expected to provide. The lower the power used by a particular engraving process, the longer the influence that transient period will have. In particular, if pluralities of single holes or engravings have to be made in a quick succession, each laser emission must provide the power that is strictly necessary for making that engraving, and then stop abruptly the emission. For instance, in the case of a half cut (i.e. a “kiss-cut”), a slightly exceeding power may cause a perforation of the support. In alternative, in the case of a through hole, the variation of the emitted power with time, for a given required power, may be detrimental due to unstable performances in case of alternation of the cut and kiss-cut operations, which would lead to an unpredictable result which can be different from what is expected.
Moreover, most systems for writing printed authenticity characters, or other authenticity signs, could be easily imitated by ill-intentioned persons. Therefore, it is desirable to mark the objects with authenticity signs that are difficult to imitate.